the nested 'service' element in the 'jar' task seems to generate a 'service' (singular) folder in which the JAR Service Provider SPI files are stored. this does not seem to work since the code (e.g. javax.imageio.spi.ServiceRegistry 1.1.3) looks for a 'services' (plural) folder.
(I am not noted for good spelling) *** This bug has been marked as a duplicate of 41201 ***